In recent years, as functional sophistication, miniaturization and weight reduction of electronic devices progress, speeding up of circuits and a high density of part mounting have further advanced. Countermeasures against EMI resulted from these trends are more emphasized. EMI means electromagnetic interference, i.e., a phenomenon that electromagnetic waves emitted by electronic devices interference with operations of peripheral electronic devices.
In general EMI countermeasures, EMI shielding components are fixed onto a portion which radiates unnecessary electromagnetic waves, so as to absorb electromagnetic wave noise, thereby suppressing radiation. In an optical module having a light emitting function and a light receiving function of signal light, used for optical fiber communication, for example, shielding components being a metallic elastic body referred to as an EMI finger are wound around itself. By inserting an optical module having an EMI finger wound thereon into a chassis, leakage of unnecessary electromagnetic waves is prevented in optical communication.
FIG. 16 illustrates an optical module having an EMI finger wound thereon. The illustrated optical module 50 is configured by a printed circuit board 51 on which a light emitting element and a light receiving element are mounted and a case 52 that covers the printed circuit board 51. Around the case 52, the EMI finger 6 being a metallic spring material is fixedly wound.
FIG. 17 illustrates a state in which the optical module 50 is inserted into a chassis. Specifically, the optical module 50 is inserted into an insertion opening referred to as a cage 31 provided on the chassis. Further, in the case where the optical module 50 performs optical transmission and reception, an optical connector C connected to an optical fiber f is inserted into the optical module 50.
Here, when the optical module 50 is inserted into the cage 31 and performs optical transmission or optical reception, unnecessary electromagnetic waves may be radiated from a gap between the cage 31 and the case 52 of the optical module 50.
Accordingly, for suppressing radiation of such unnecessary electromagnetic waves, the EMI finger 6 is wound around a predetermined portion of the optical module 50 so as to come in contact with a leading edge side of the cage 31. At the time when the optical module 50 having the EMI finger 6 wound thereon is inserted into the cage 31, and the EMI finger 6 comes in contact with the cage 31, external radiation of unnecessary electromagnetic waves is suppressed.
As a conventional technology, in Japanese Laid-open Patent Publication No. 2006-106680, the following technology is disclosed. That is, the sleeve of the tip section of a light-emitting element module or a light-receiving element module is made of ceramics, and a holder and an adaptor are made of metals. As a result, the EMI is suppressed. Also, in Japanese Utility Model Application Publication No. 05-20014, disclosed is a technology in which applied voltage of a piezoelectric element is controlled to adjust an optical axis.
As described above, the EMI finger 6 being an electromagnetic wave shielding material is heretofore fixed around the optical module 50. The EMI finger 6 of the optical module 50, when inserted in the chassis, comes into contact with the cage 31 of the chassis, thereby preventing the radiation of electromagnetic waves.
However, when repeating the insertion/extraction of the optical module 50, the EMI finger 6 undergoes plastic deformation (deformation in which the EMI finger 6 gets deformed by external force, and its shape fails to return to original even after the force is removed), so that a contact between the EMI finger 6 and the cage 31 is degraded in some cases. There arises a problem that when the contact between the cage 31 and the EMI finger 6 is degraded, unnecessary electromagnetic waves are radiated from a leading periphery of the cage 31, and as a result, the EMI increases.
For the purpose of correcting the above-described plastic deformation of the EMI finger 6, which occurs across the ages, countermeasures of increasing spring pressure (elastic constant) of the EMI finger 6 to prevent the plastic deformation from easily occurring, and increase a contact pressure (contact friction) are considered to be adopted.
However, an optical module must be pursuant to specifications referred to as multi source agreement (MSA) in which mechanism and electrical specifications are specified. With regard to an insertion/extraction force of the optical module, specifications of the insertion/extraction force specified in the MSA must be satisfied. Therefore, countermeasures of increasing spring pressure of the EMI finger 6 to increase the contact pressure between the optical module and the cage 31 cannot be simply adopted.